How does blender work?
I think the main thing you will want to learn first are the basics of a computer graphics development. This generally means learning the basics of OpenGL programming. You will also need to go over some basic trigonometry and linear algebra for this. Once you understand these basics it should be a lot easier to just google for individual topics and find explanations. 

I learnt most of this at my university introduction to computer graphics course, so maybe you could try looking at an online course covering these topics. I noticed a couple of online courses that look like they cover similar ground, although I haven't tried them myself, so I don't know the teaching quality:
https://www.coursera.org/course/webgl
https://www.edx.org/course/computer-graphics-uc-san-diegox-cse167x-1

When you get more familiar with the basics, you can try looking at blender's source code as well. I wouldn't recommend that for a complete beginner though, as it can be quite hard to follow in places.